Изображения страниц
PDF
EPUB
[merged small][merged small][graphic]

FIG. 6.-SPECTRUM PHOTOGRAPH OF LIGHTNING FLASH, JUNE 18, 1905.

[graphic][merged small]
[graphic]

FIG. 8.-SPECTRUM PHOTOGRAPH OF SPARK FROM STATIC MACHINE.

[graphic][merged small]

THE TANTALUM LAMP.

By Dr. W. VON BOLTON and Dr. O. FEUERLEIN.

PART I.-By Dr. W. von BOLTON.

Whilst the carbon-filament incandescent lamp remained for nearly two decades the sole representative of glow-lamp manufacture, progress was being quietly made in this art. The firm of Messrs. Siemens & Halske has for many years been working at a solution of the problem of an economical incandescent lamp, and arrived, some time ago, at the fundamental principle that the visible part of the radiation of an incandescent body increases progressively with its temperature. This warrants the postulate that the most economical lamp will be that whose incandescent material will withstand the highest temperature.

Messrs. Siemens & Halske had arrived at this conclusion and charged me several years ago with the task of discovering a material which should have a melting point considerably above the temperature at which incandescent lighting becomes highly economical, so that filaments made of such a material would not melt or disintegrate at that temperature. Whilst our laboratory work, founded upon this idea, was going on, the first two advances in incandescent lighting were made public, one being the "Nernst" and the other the "Osmium" lamp.

There are certain metals the melting points of which are known to be considerably above 2,000° C., and the task resolved itself into finding one which, while fulfilling the above requirement, could be easily worked to form a filament, and not be very rare or difficult to proIt was early observed that brown vanadium pentoxide, which, according to Berzelius, does not conduct electricity, is, as a matter of

cure.

Translation (through the courtesy of Mr. Alexander Siemens) of a paper read before the Elektrotechnischer Verein of Berlin on January 17, 1905. Reprinted, by permission, from the Electrician, London, No. 1393, Vol. LIV, No. 15, January 27, 1905.

Cf. W. Siemens in Elektrotechnische Zeitschrift, Vol. IV., p. 107, 1883.

fact, a conductor even when cold. This observation induced me to try whether vanadic acid could not be electrolytically decomposed. In this I succeeded, but the melting point of the vanadium obtained proved too low for the purpose in view. Since the metals niobium and tantalum are members of the vanadium group, niobium having an atomic weight double that of vanadium, while the atomic weight of tantalum is double that of niobium, it was thought that one or both of these metals might prove to have the desired qualities. On experimenting with niobium on the lines adopted for vanadium it appeared that this metal has a considerably higher melting point than that of vanadium, but not, however, sufficiently high. Moreover, some of the niobium filaments which I made had a very strong tendency to break up when heated by the electric current.

Tantalum was tried next. I reduced potassium tantalo-fluoride in the manner prescribed by Berzelius and Rose and found that the finely divided tantalum so produced became fairly coherent on rolling, so that by this treatment metallic strips of it could be made. It was also attempted to work tantalum oxide into the shape of a filament by mixing it with paraffin and to reduce it directly into the form of a metallic thread. In these experiments there was observed for the first time a minute globule of molten tantalum, and this globule was of sufficient toughness to permit hammering and drawing into wire. Following out this observation, tantalum powder was melted in a vacuum, and then it was found that the highly heated metal parted with the gases it contained. In this manner I produced my first filaments of pure metallic tantalum, which were, however, very small. When these had been used in lamps with promise of good results, an attempt was made to devise a definite process of purification. The potassium tantalo-fluoride was reduced to metallic powder; this powder contains a small proportion of oxide and of hydrogen which is absorbed during the reduction. When the powder was melted in a vacuum the oxide and absorbed gas disappeared and a reguline metal remained; on carefully remelting this it became so pure that no appreciable impurities could be detected in it.

The chemical properties of this pure tantalum are very remarkable, and some of them are of such a nature as to lead me to suppose that nobody other than myself has ever had metallic tantalum in his hands. When cold, the material strongly resists chemical reagents; it is not attacked by boiling hydrochloric acid, aqua regia, nitric acid, or sulphuric acid, and it is also indifferent to alkaline solutions; it is attacked solely by hydrofluoric acid. Following the behavior of steel, when heated in the air it assumes a yellow tint at about 400° C., and the tint changes to dark blue when the tantalum is exposed for some time to 500° C., or for a shorter time to 600° C. Thin wires of the substance burn with low intensity and without

any noticeable flame when ignited. It absorbs hydrogen as well as nitrogen with great avidity, even at a low red heat, and forms with them combinations of a metallic appearance, but rather brittle. It combines with carbon very easily, forming several carbides which, as far as they are at present known, are all of metallic appearance, but also very hard and brittle. The product which Moissan thought to be tantalum was clearly a carbide of this nature or an alloy of a carbide with pure tantalum, for Moissan himself stated that his metal still contained one-half per cent of carbon. Considering the high atomic weight of tantalum (183) it is obvious that a very small quantity of carbon suffices to carburize a relatively large quantity of tantalum. This view of the constitution of Moissan's product is confirmed by the properties he ascribed to the metal-namely, specific gravity 128, great hardness and brittleness. These are not properties of pure tantalum. When in the form of powder, still containing, as previously stated, oxide and hydrogen, the specific gravity of my material is about 14; when purified by fusion and drawn into wire it has a specific gravity of 16.8. It is somewhat darker than platinum, and has a hardness about equal to that of mild steel, but shows greater tensile strength than steel does. It is malleable, although the effect of hammering is relatively small, so that the operation must be rather long and severe to beat the metal into a sheet. It can be rolled, as well as drawn, into very fine wire. Its tensile strength as a wire is remarkably high, and amounts to 95 kilograms per square millimeter, while the corresponding figure for good steel is 70 to 80 kilograms, according to Kohlrausch.

The electrical resistance of the material at indoor temperature is 0.165 ohm for a length of 1 meter and a section of 1 square millimeter (specific conductivity as compared with mercury 606). The temperature coefficient is positive and has a value of 0.30 between 0° C. and 100° C. At the temperature assumed by the incandescent filament in the lamp at 15 watts per candlepower, the resistance rises to 0.830 ohm for a length of 1 meter and a section of 1 square millimeter. The coefficient of linear thermal expansion between 0° C. and 60° C. is 0·0000079, according to experiments made by the Imperial Normal-Aichungs commission. Fusion is preceded by a gradual softening, which appears to extend over a range of temperature of several hundred degrees. The specific heat is 0.0365, so that the atomic heat is 664, which is in accord with the law established by Dulong and Petit.

PART II.-By Dr. O. FEUERLEIN.

The results of the work carried out in our chemical laboratory, as described by Doctor von Bolton in the first part of this paper, were, of course, of the utmost interest to our incandescent lamp manufac

« ПредыдущаяПродолжить »